A non-aqueous electrolyte secondary battery comprises a positive electrode, a negative electrode and a non-aqueous electrolyte. The positive electrode comprises: a positive electrode mixture layer comprising a positive electrode active material, a conductive agent, a binder and a thickening agent; and a positive electrode current collector carrying the positive electrode mixture layer. The negative electrode comprises: a negative electrode mixture layer comprising a negative electrode material, a binder and the like; and a negative electrode current collector carrying the negative electrode mixture layer.
The positive electrode mixture layer and negative electrode mixture layer are formed by applying, to the respective current collectors, pastes each comprising a mixture of the starting materials and a polar dispersion medium. An aqueous dispersion medium or a polar organic dispersion medium is used as the polar dispersion medium. However, the use of an aqueous dispersion medium causes water to remain in the electrode plates, and thus is likely to induce generation of hydrogen gas inside the battery. For this reason, a polar organic dispersion medium such as N-methyl-2-pyrrolidone (hereinafter referred to as NMP) is more often employed than an aqueous dispersion medium.
Heretofore, poly(vinylidene fluoride) (hereinafter referred to as PVDF), polytetrafluoroethylene (hereinafter referred to as PTFE) and the like are employed as the binder for the positive electrode mixture layer. From the viewpoint of making PVDF, PTFE and the like to sufficiently function as the binder, they are added in a large amount to the positive electrode mixture. However, when the positive electrode contains a large amount of the binder, an electroconductive network is not sufficiently formed in the positive electrode. Accordingly, it is also necessary to use a large amount of a conductive agent typified by carbon black. Naturally, there is a certain limit to increase the capacity of a battery by improving the active material density of the positive electrode mixture layer. The active material density as used herein is calculated by dividing the weight of the positive electrode active material contained in the positive electrode mixture layer by the volume of the positive electrode active material layer.
The present inventors have discovered that it is effective to employ a particulate binder, which is insoluble in a dispersion medium of a paste but capable of being dispersed therein, in order to improve the active material density. For example, rubber particles comprising a copolymer of 2-ethylhexylacrylate, acrylic acid and acrylonitrile may be used as the above-mentioned binder. In this case, the amount of the binder can be significantly reduced, so that it becomes possible to improve the active material density of the positive electrode.
Herein, the particulate binder is insoluble in the dispersion medium, and therefore does not function to control the viscosity of the paste. Accordingly, it is necessary to add a small amount of a thickening agent with the paste. The thickening agent is required to have a certain polarity so that it can be dissolved in the dispersion medium of the paste. For example, a polar resin, such as an acrylic resin having a hydroxyl group, may be employed as such thickening agent.
Since the polar resin has high affinity with a polar dispersion medium such as NMP, it is possible to control the properties of the paste with a small amount of the polar resin. The polar resin, however, is likely to retain water because of its high hydrophilicity. Water is difficult to be removed by a drying step during the production process of an electrode plate, and thus remains inside the positive electrode even after the fabrication of the battery. The residual water may result in generation of hydrogen gas at the negative electrode side during charging of the battery, causing problems such as the swelling of the battery.